The present invention relates to a gas detector with an acoustic measuring cell and a selectively adsorbing surface.
Essentially, four possibilities are available for detecting gases: In the measurement with sampling, constituents of the gas phase are collected on an adsorber. For analysis, the adsorbed constituents are transferred into the gas phase again. This method permits enrichment of the constituents of interest. The collection on adsorber tubes with subsequent thermal desorption (TD), use gas chromatography (GC) for fractionation and a mass-selective detector (MSD) or flame ionization detector (FID) for detection, which are cited by way of example. The advantages are the selectivity and the low detection limit. However, real-time measurements cannot be carried out with chromatographic methods.
With direct measurement in the gas phase (e.g., fluorescence, IR spectroscopy, UV/VIS spectroscopy, photoacoustics), enrichment as well as pre-separation of the constituents are omitted. These disadvantages—in particular with reference to selectivity—can be expensive to overcome.
Another possibility for detecting gases is through measurement with chemical conversion of the analyte. The conversion can be carried out on a surface or in a solution. Two methods can be distinguished:
a) Those in which a reactant has to be fed to the system. These systems are maintenance-intensive due to the consumption of chemicals and thus not economically justifiable for many applications.
b) Those in which the reactant is atmospheric oxygen. The differentiation of the constituents of interest from the matrix is carried out by a more or less selective reaction, i.e., the selective oxidation at certain heated metal oxide surfaces. Here, low selectivity can also be expensive to overcome.
Furthermore, a measurement by changing physical properties on or at a surface is known. For example, quartz balances and surface-wave detectors are used for this purpose. Physical detectors, however, are problematic because of their low selectivity.
DE 199 13 220 C2 describes a collecting medium that has a selectively adsorbing surface. Substances to be examined can be adsorbed on such a surface. The substances can then be desorbed again for a subsequent measurement. This desorption can also be carried out thermally. DE 199 13 220 C2, for example, mentions, inter alia, photoacoustics as a possible measurement method for determining which substance has been desorbed. Consequently, DE 199 13 220 C2 discloses a measuring device in which the gases desorbed from an adsorbing surface can be examined photoacoustically. The desorbed gases are excited by a light source, normally a laser, in order to generate a photoacoustic signal through absorption. WO 03/026774 A1 also discloses a device, which can photoacoustically examine gases desorbed from a surface.